The present invention relates to methods for fabricating a semiconductor device including a gate insulating film of high dielectric material.
With recent progress in technologies for increasing the packing density and the speed of a semiconductor device, MOSFETs in the device have been increasingly miniaturized. However, if a gate insulating film thereof is made thinner in accordance with this miniaturization, the problem of, for example, an increase in a gate leakage current resulting from a tunnel current becomes apparent. To solve this problem, a technique is studied in which use of a metal oxide such as hafnium oxide (HfO2) or zirconium oxide (ZrO2), that is to say, a high dielectric material for the gate insulating film accomplishes a reduced equivalent silicon oxide (SiO2 film) thickness EOT (Equivalent Oxide Thickness) and simultaneously makes the physical thickness of the film greater. An example of the technique is disclosed in Extended Abstracts of the 2000 International Conference on Solid State Devices and Materials (Japan, Aug. 29, 2000), pp. 228-229, which uses ZrO2 for the gate insulating film.
However, it is reported that in the case of the gate insulating film made of metal oxide such as ZrO2, an interface layer (for example, a silicate layer) is created between the surface of a silicon substrate and the gate insulating film of a high dielectric constant. The interface layer has a lower dielectric constant than the metal oxide as the high dielectric material, so that the occurrence of the interface layer disadvantageously reduces the effective dielectric constant of the gate insulating film.
In a gate insulating film of a high dielectric material, it is proposed that a composite film or a film with a gradient composition using various types of metal oxide films be employed as means for enhancing the effective dielectric constant and the heat-resisting properties of the gate insulating film. However, because of the inter-diffusion of these metal oxide films in the deposition or oxidation, the proposal does not always provide a gate insulating film exactly as designed with respect to the thickness of the interface layer and the composition profile of the composite film or the like in the thickness direction.